How do I assign a material to an Archipack object?
Archipack can take advantage of a predefined material library.
If the default material library is missing, you get dumb white materials assigned.
Please take a look at https://github.com/s-leger/archipack/wiki/Materials in order to setup your default material library.
The aswer of @light-pankakes remains still valid when it comes to setup new materials.